Dear teachers, I'd like to know which one is correct of the following sentences , and why?

1, December is the last month of the year.

2, December is the last month of a year.

Thank you in advance.
With Ordinal Numerals like the first, the second, the third, .... the last we use a definite article, Ong.
December is the last month of the year.
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It arouse my interest that isn't that December is the last month of any year and not a definite year.

So how can we use December is the last month of the year?

Could anybody explain it to me?

Victory Ong
 Fandorin's reply was promoted to an answer.
So is the problem that you're finding 'THE' year. The 'the' in December is the last month of the year can happen in two ways. It can happen if part is specified (definite) but the whole is unspecified, but universal. So we would say: December is the best month of THE year (every year). Hanukkah is the holiest week of THE year (every year). Christmas is best day of THE year (every year). Wednesday is the most difficult day of THE week (every week). 5 o'clock is the best hour of THE day (every day). OR EVEN bacon is the best part of THE pig, the breast is the best part of THE chicken.

This happens because 'the year' is not indefinite. While it may be unspecified it IS defined. It's every single year that has ever come or will ever come. There is no particular year for which this is not true, even if it does not refer to a particular year. Indefinite means that it can be ANY year, you may not know which one. Every year, you DO know which. In this sense, "year" is abstracted. We're talking about the idea of 'year'.

Also be aware that some expressions can go either way depending on what the speaker has in mind:
12 months in A year (any year) vs. 12 months in THE year (every year)
Seven days in A week (any week) vs. THE week is comprised of seven days (every week)
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did you get the first answer?